Heating devices, also called hot water storage tanks or boiler, in which cold water enters at the bottom into an inner tank through an inlet tube and the hot water is discharged through an outlet arranged at the upper end of the inner tank due to displacement, are well-know. Furthermore, Swiss publication CH-A 597,447 discloses a heating device which has a heating chamber, in which the cold water flowing in is heated in the manner of an immersion heater, and a storage chamber following it in the direction of flow, in which the heated shower water is kept ready for use. This subdivision of the inner space of the heating device is said to ensure uniform water temperature in the storage chamber. The storage chamber and the heating chamber are subdivided by a horizontal tube section, which permits counterflow of the water. Correspondingly the outlet for the water from the storage chamber and the inlet for the cold water are arranged on the same side of the tube section. This heating device has a cylindrical outer shape and is therefore suitable for installation in a correspondingly tubular installation element, in which the other devices for the bidet are also accommodated, and which is installed in the hollow projection of the toilet seat. In contrast, this heating device is associated with the difficulty that the outer shape is practically completely defined by the above-mentioned design. Toilet seats with such a heating device correspondingly have a relatively tall projection, which often fails to meet the aesthetic requirements imposed on modern installations. Especially a toilet seat with a larger and flatter top side and a correspondingly lower projection would be desirable.